To know those who unfollowed you on Instagram, it takes a tremendous amount of time. First, you check your follower's list; it would be great if you knew the numbers. This will help you know if your followers have decreased in number. Then you can check your list to know who is no longer following you, which will take some time. However, with the introduction of some apps, you can tell when someone unfollows your account. An example of such apps is Instrack.
This app tracks gained and lost followers since your last refresh and those who you follow and are not following you back, and those who follow you and you did not follow back. It gives you opportunities to view these statistics. It also requires charges to see who blocked you. Another app is Instafollow; this app shows people who unfollowed you and also your followers on Instagram. It allows you to see who unfollowed you, the people not following you back, and the people who blocked you for free.
In the world of social media where you can follow and unfollow people at random, it may help you to be able to see who has unfollowed you. You can use this as an opportunity to either reach out to them and find out their reasoning or check out their profile and see if maybe your content just is not in the same category as what they like to see. So there is no way on Instagram to see who has unfollowed you.
There is a simple app that is a great tool just for this purpose. It is called Unfollowgram. In order to use it all, you need to do is to connect it to the Instagram you have and it will show you instantly who unfollowed you. There are simple instructions to follow in the app in order to set this up.